The present invention relates to a single-stage fully differential operational amplifier having a high open loop gain, and constructed by MOS technology and comprising components (transistors) for a transconductance stage, load and bias components for the aforementioned stage and a circuit CMFB for stabilizing the quiescent output voltage, i.e., the output voltage of the operational amplifier when the differential input voltage is zero. The use of operational amplifiers in integrated circuits in MOS technology has become increasingly common in recent years, owing to the need to integrate analog circuits and sub-systems in this technology.
A summary of design techniques for MOS integrated operational amplifiers (NMOS or CMOS) is given e.g. in the article "MOS Operational Amplifier Design--A Tutorial Overview" by P. R. Gray and R. G. Mayer, in "IEEE Journal of Solid-State Circuits", Vol., SC-17, No. 6, page 969, December 1982.
The main requirements for an operational amplifier ("OPAMP") integrated into a monolithic MOS circuit (See FIG. 1, for example, where V.sub.REF and V.sub.RIF are two fixed voltages which are not necessarily different from one another, and A is the open-loop gain of the amplifier) are as follows:
high open-loop gain PA0 short settling time PA0 capacity to drive capacitive loads PA0 high rejection to the supply voltage "PSRR", Power Supply Rejection Ratio and PA0 low input-referred noise. PA0 occupation of small area of silicon PA0 ease of interconnection with other parts of the integrated circuit PA0 large swing of output voltage with low harmonic distortion and PA0 limited power dissipation. PA0 (1) An improvement in the maximum swing of the effective output voltage; PA0 (2) A reduction in the harmonic distortion in the output signal (more particularly in the harmonic distortion due to even harmonics) and PA0 (3) An increase in the value of the PSRR. PA0 open-loop gain: high PA0 settling time: depends on the load capacitance but in any case is not larger than for a two-stage amplifier PA0 driving capability of capacitive loads: good, without the need to add a compensating capacitor of equal value to the load capacitance PA0 PSSR excellent, in that the output voltage is taken between the OUT+ and OUT- junctions without being referred to a fixed voltage PA0 input-referred noise: fairly low (of the same order as for the two-stage amplifier) PA0 area of silicon occupied: less than for the two-stage amplifier PA0 ease of interconnection: depends on the number of different bias voltages needed for proper operation of the amplifier PA0 output voltage swing: good, in that the output voltage is taken between two complementary outputs. The maximum swing depends on the bias voltages and the dimensions and threshold voltage of the transistors making up the "cascode". PA0 power dissipation: depends on the required settling time and is substantially fixed by the bias voltage V.sub.BIAS1.
Other requirements which are particularly useful for integrated operational amplifiers are:
A recently-established method of design uses operational amplifiers with a differential output ("fully-differential" or "double-ended" amplifiers) where the output voltage is taken not between the individual amplifier output and a fixed reference voltage (e.g. ground or another voltage generated inside the integrated circuit) but between the two amplifier output. A fully-differential amplifier is designed so that the two voltage signals are symmetrical relative to a reference voltage V.sub.REF (See FIG. 2, for example).
A main difference between single-ended and fully-differential operational amplifiers is that the latter do not have a reference node common to the input and the output of the operational amplifier.
An essential advantage of the fully-differential approach is that the following improvements are obtained:
A typical problem in this class of amplifiers is the need to design circuitry which fixes the "quiescent" voltage of the two outputs (i.e.--the voltages present at the two outputs in the absence of signals applied to the inputs) at a value which ensures a symmetrical, maximum swing of output signals (the quiescent output voltage is in general V.sub.REF, see FIG. 2).
Frequently, (typically when the circuit is supplied with a single voltage) the quiescent voltage of the two inputs is also fixed by circuitry which optimizes the input level with regard to the circuit requirements of the amplifier. (This voltage in general is V.sub.RIN, see FIG. 2).
One method used for designing "fully-differential" amplifiers uses two stages in cascade in order to meet the basic requirement of high gain, since each stage by itself has insufficient gain for the desired application.
The main and well-known disadvantage of this approach is that a two-stage amplifier requires the use of a rather higher compensation capacitor (whose size increases with increasing load), disposed between the first and second stage, to ensure the stability of the system in which the amplifier is inserted. This also appreciably increases the area occupied by the amplifier.
An alternative known method of design, which is becoming increasingly successful, uses a single amplifier stage which by itself has a sufficiently high gain to meet requirements. A main advantage of this type of amplifier is that it eliminates the need for a compensating capacitor.
Basically, a single-stage amplifier is an amplifier which has only one stage having a high transconductance gm and performing a transfer function EQU I.sub.OUT =gmV.sub.IN ( 1)
where I.sub.OUT represents the output signal current of the stage and V.sub.IN represents the input signal voltage. In the complete amplifier, voltage amplification is obtained as a result of the drop produced by I.sub.OUT across an output impedance Z.sub.OUT, as a result of which the open-loop voltage gain of the amplifier, A, is: EQU A=gmZ.sub.OUT ( 2)
One example of this type of single-stage approach uses a stage known as a "folded cascode" (see FIG. 3, and page 979 of the previously cited article, for example). In FIG. 3 and the subsequent figures, the NMOS and PMOS transistors are understood as having their "body" electrode connected to the more negative and the more positive supply voltage respectively, except where otherwise indicated. In the circuit in FIG. 3, more particularly, any of the transistors can have its "body" electrode connected to its "source" electrode if specially required.
In the conventional circuit in FIG. 3, the high-transconductance components are transistors M2 and M3. Transistors M1, M4, M5, M7-M12 serve to meet the load and bias requirements of the stage. Transistor M6 is for fixing the quiescent voltage of the outputs.
In the diagram in FIG. 3, a rectangle indicates the circuit unit for stabilizing the quiescent output voltage (CMFB=common mode feedback). This circuit unit can be constructed in different ways, depending on circuit requirements. For example, the feedback path can be embodied by continuous-time or sample-time circuitry. With regard to the previously-stated requirements, the amplifier shown in FIG. 3 behaves as follows:
Even with these advantages, single-stage fully-differential operational amplifiers available at present are not free from limitations and disadvantages. For example, with their present structure they cannot easily satisfy the increasingly high requirements regarding compactness; ease of interconnection with the other parts of the integrated circuit in which the amplifier is inserted; suitability for integration into circuits with a single supply, even at a low-voltage; reduction in dissipated power; possibility of integration on minimum area of silicon with minimum number of bias interconnection lines with the rest of the circuit, etc.